Practical Boat Owner - - Learning From Experience -

stu davies com­ments

The mo­tor­boat in ques­tion is fit­ted with twin Perkins 4108 en­gines mated to stern drives. Th­ese en­gines were built in the 1960s. The Perkins 4108 is a good en­gine, solid, de­pend­able and thou­sands of them were built and used in boats, trac­tors, fork lifts, gen­er­a­tors – and in fact any­thing that needed a de­pend­able work­horse en­gine with spares read­ily avail­able world­wide.

The 4108 was marinised by a few man­u­fac­tur­ers by fit­ting sea­wa­ter­cooled heat ex­chang­ers/in­ter­cool­ers on the port side of the en­gine where the ex­haust comes out of the cylin­der head.

The in­jec­tor pump is fit­ted mid­way down the en­gine on the port side, un­der­neath the heat ex­changer.

Like all marinised en­gines, the heat ex­changer cool­ing sea­wa­ter is in­jected into the ex­haust el­bow aft of the heat ex­changer from where it goes along the ex­haust pipe to cool it and that is where is­sues can arise.

Over the years car­bon and salt can build up in the ex­haust el­bow where the waste cool­ing sea­wa­ter is in­jected, lead­ing to block­ages. Also the hot sea­wa­ter com­bined with the hot ex­haust gases pro­vides the per­fect en­vi­ron­ment for cor­ro­sion to take place in the el­bow.

I sus­pect from Roger’s story that the el­bow/s were on their last legs – se­verely cor­roded and just wait­ing for

the most per­fect, in­op­por­tune mo­ment to let go. Roger refers to the over­heat­ing on the port en­gine and of course that would then lead to low oil pres­sure read­ings as well. This could have been caused by a loose or bro­ken con­nec­tion, a bad wa­ter pump im­peller or some­thing sim­i­lar.

The fact it stopped and wouldn’t restart probably in­di­cates that the en­gine seized from over­heat­ing.

The ref­er­ence to flames com­ing out of the ‘loose man­i­fold’ and the sub­se­quent sur­vey in­di­cates that the ex­haust el­bows per­fo­rated, and ex­haust flames would have been com­ing out of there and the waste raw cool­ing wa­ter would also have been com­ing in to the boat from the same place.

The ref­er­ence to the ‘glow­ing red’ in­jec­tor pump I think can be put down to the stress­ful sit­u­a­tion they were in. As ex­plained pre­vi­ously the in­jec­tor pump is fit­ted on the port side of the en­gine un­der the heat ex­changer. Even on a per­fect en­gine the heat ex­chang­ers get se­ri­ously hot! Try­ing to reach past it and un­der­neath it to reach the stop lever would not be some­thing I’d like to do!

Nor­mally the ex­haust gases go out of the en­gine down the steel el­bow and then in to a si­lencer, (some of which are plas­tic) and then rub­ber hose to exit the boat.

Could this start a fire?

The raw wa­ter in­jec­tion is at the el­bow and the wa­ter both cools the plas­tic si­lencer and rub­ber hose and muf­fles the ex­haust sound. If the el­bow cor­rodes through, you get the sit­u­a­tion Roger de­scribes with the raw wa­ter now go­ing into the boat, and flames com­ing out of the man­i­fold. Can this cause a fire?

With no cool­ing wa­ter and the plas­tic si­lencer and rub­ber ex­haust hose sub­ject to the ex­haust gases, which as he said are seen as flames, the si­lencers melt and the rub­ber hose chars. At this point there is a real pos­si­bil­ity that fire can take hold.

Even if it doesn’t come from there, de­pend­ing on the prox­im­ity of flammable ob­jects, it could start an en­gine room fur­ni­ture fire.

This sad story is a per­fect ex­am­ple of why, at the very least, you need a knowl­edge­able per­son to cast an eye over things first and then con­sider hav­ing a pro­fes­sional sur­vey done.

‘Over the years car­bon and salt can build up in the ex­haust el­bow where the waste cool­ing sea­wa­ter is in­jected, lead­ing to block­ages’

The con­se­quences of an over­heat­ing en­gine were not quite as se­ri­ous as this on Anne’s boat – but could well have been

PBO en­gine ex­pertStu Davies worked in the oil and gas in­dus­try for much of his ca­reer and now di­vides his time be­tween Por­tu­gal, where he keeps his Beneteau 381, and his home in Wales.

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